1. Field of the Invention
The present invention relates to a sheet conveying apparatus that separates sheets one by one from a sheet housing unit housing a plurality of sheets and conveys the separated sheet. The present invention also relates to an image reading apparatus and an image forming apparatus that include the sheet conveying apparatus.
2. Description of the Related Art
A conventional image reading apparatus that is used as an image reading unit or a scanner in a copying machine includes an image reading apparatus that reads images by using what is called a sheet-through system, that is, that reads images on sheet originals by using an image reading unit fixed to the apparatus while conveying the sheet originals at a predetermined speed. The sheet-through type image reading apparatus is configured to read images while conveying originals, so that the apparatus of this type has an advantage in increasing the productivity compared to an image reading apparatus that stops originals and performs exposure on the stopped originals (book type).
The sheet-through type image reading apparatus includes an automatic document feeder as a sheet conveying device that conveys sheets one by one from an original table, which is a sheet housing unit on which a plurality of originals is stacked, to a reading position, at which an image reading unit reads images from the originals. The automatic document feeder needs to prevent multiple feeding, in which two originals that are successively conveyed (hereinafter, described as a preceding original and a next original) at least partially overlap each other while continuously reading originals.
Meanwhile, some of the conventional pieces of image reading apparatus include a document conveying unit that conveys originals to the read position; and a pick-up roller as a document pick-up unit that picks up one of the originals placed on the original table and feeds the original toward the document conveying unit. The pick-up roller comes into contact with the top surface of a topmost one of the originals placed at a predetermined position on the original table in the conveying direction and rotates in order to apply a conveying force to the topmost original so that the topmost original is conveyed toward the document conveying unit. Such an automatic document feeder also includes a separating unit that, if a next original is conveyed toward the document conveying unit together with a preceding original and overlaps with the preceding original to which the conveying force is applied by the pick-up roller, separates the next original from the preceding original so that only the preceding original can be conveyed toward the document conveying unit. Multiple feeding is prevented by the separating unit that conveys originals one by one toward the document conveying unit.
Recently, operational efficiency has been required, and demands for an increase in the productivity in the automatic document feeder, i.e., an increase in a document read rate, are more and more increasing.
To ensure a certain level of the productivity in the automatic document feeder, it is necessary to set an interval between the preceding original and the next original that are successively conveyed (hereinafter, described as a sheet interval) to be within a predetermined range. Therefore, there is a known technology for controlling a timing of feeding the next original with reference to a detection signal of a trailing-end detection sensor that detects passage of the trailing end of the preceding original through a predetermined position. As the trailing-end detection sensor, a sensor using a reflective or transmissive photo sensor is known that detects presence or absence of an original by applying light to the surface of the original in order to detect passage of the trailing end of the original through a predetermined position.
However, with the configuration in which the passage of the trailing end of the preceding original through the predetermined position is detected by using the reflective or transmissive photo sensor, if the preceding original and the next original overlap each other at the predetermined position, it is impossible to detect the passage of the trailing end of the preceding original. The reasons will be described below.
The reflective or transmissive photo sensor detects that the original is passing through the predetermined position while detecting reflected light from the original at the predetermined position. After the trailing end of the original passes through the predetermined position, because no light is reflected by the original, the reflective or transmissive photo sensor detects passage of light through the predetermined position, thereby detecting that the trailing end of the original has passed through the predetermined position. With the trailing-end detection sensor using a photo sensor, when the preceding original and the next original overlap each other at, the predetermined position, the next original is present at the predetermined position after the trailing end of the preceding original passes through the predetermined position and light from the photo sensor is reflected by the next original. Therefore, even when the trailing end of the preceding original passes through the predetermined position, the photo sensor detects that the preceding original is passing through the predetermined position and fails to detect that the trailing end of the preceding original has passed through the predetermined position.
In some cases, the next original being conveyed toward the original conveying unit together with the preceding original may be present at the separating unit. Therefore, the configuration in which the trailing-end detection sensor using the reflective or transmissive photo sensor is arranged in the separating unit may be incapable of detecting the trailing end of the preceding original. Consequently, in the conventional technology, it is necessary to arrange the trailing-end detection sensor at a position at which the trailing-end side of the preceding original and the leading-end side of the next original certainly do not overlap each other, e.g., at a position separated from the separating unit by a predetermined distance downstream of the separating unit in the conveying direction. If the trailing-end detection sensor is separated from the separating unit by a predetermined distance downstream of the separating unit in the conveying direction, timing of feeding the next original is delayed by an amount corresponding to the distance, which impedes the high productivity.
To increase the productivity, the trailing-end detection sensor may be arranged at a position at which the passage of the trailing end of the preceding original can be detected at the earliest possible time. A position on the upstream side of the separating unit may be the position at which the passage of the trailing end of the preceding original can be detected at earlier timing than the conventional timing. As a configuration that allows detection of the passage of the trailing end of the preceding original on the upstream side of the separating unit, Japanese Patent No. 3618898 and Japanese Patent No. 3397606 disclose a configuration in which a roller member is provided that comes into contact with the top surface of originals stacked on a original table and a change in the speed of the roller member is detected in order to detect passage of the trailing end of the preceding original. This configuration can be regarded as valuable in detecting the presence or absence of movement of an original in the conveying direction so as to detect passage of the trailing end of the preceding original.
However, in the configuration disclosed in Japanese Patent No. 3618898 and Japanese Patent No. 3397606, the roller member that comes into contact with the top surface of the originals on the original table protrude on the upstream side of the pick-up roller. Therefore, there is a problem in that such a configuration is not practically useful because the original setting capability of the original table may be largely reduced and the operability may be reduced accordingly.
In view of this, there is a demand for an automatic document feeder that can detect passage of the trailing end of the preceding original through a predetermined position at earlier timing and that can increase the productivity without, reducing the operability.
The demand for detection of the passage of the trailing end of the preceding original through a predetermined position at earlier timing is not limited to the automatic document feeder. In any sheet conveying devices that convey sheets one by one from a sheet housing unit housing a plurality of sheets to a conveyance target position, it is needed to detect passage of the trailing end of the preceding original through a predetermined position at earlier timing and increase the productivity.